Thin lightshield process for solid-state image sensors

ABSTRACT

An image sensor includes a substrate having photosensitive areas; an insulator spanning the substrate; and a first and second layer of a multi-layer metallization structure, wherein the first layer forms light shield regions over selected portions of the photosensitive area as well forming circuit interconnections and barrier regions to prevent spiking into the substrate or gates at contacts in the non-imaging area; and the second layer spanning the interconnections and barrier regions of the first layer only over the non-imaging areas and the second layer overlays edges of the first layer.

FIELD OF THE INVENTION

The invention relates generally to the field of solid-state imagesensors, and more particularly to the process of forming a lightshieldand the interconnection layers for a solid-state image sensor.

BACKGROUND OF THE INVENTION

Image sensors are made of an array of pixels. Within each pixel, someregions are specifically designed to be photosensitive, and otherregions are protected from light by a lightshield. Regions are protectedfrom light because light absorbed in these protected regions causesdegraded performance through mechanisms such as color crosstalk, smear,or reduced blooming control.

In U.S. patent application Ser. No. 10/641,724, filed Aug. 15, 2003,entitled “Light Shield Process For Solid-State Image Sensors,” by EricG. Stevens, a thin lightshield process is described for providing alightshield from one of the layers of a bi-layer metallization process.The aluminum layer in this process is usually patterned with achlorine-based plasma chemistry which leaves chlorine-containing residueon the wafers after the etch. Further, this residue may react with thealuminum or TiW, especially where the aluminum and TiW meet, causingcorrosion of these films, and degradation of their electrical propertiesor optical light-shielding properties. In addition, U.S. patentapplication Ser. No. 10/641,724 requires that the etch of the bottomlayer of the bi-layer metal be masked in some regions by the top layerof the bi-layer metallization. This requirement may restrict the use ofcertain metals for the bi-layer metallization.

Consequently, a need exists for producing image sensors that overcomethe above-described drawbacks.

SUMMARY OF THE INVENTION

The present invention is directed at overcoming the problems describedabove. The invention resides in an image sensor comprising (a) asubstrate having photosensitive areas; (b) an insulator spanning thesubstrate; and (c) a first and second layer of a multi-layermetallization structure, wherein the first layer forms light shieldregions over selected portions of the photosensitive area as wellforming circuit interconnections and barrier regions to prevent spikinginto the substrate or gates at contacts in the non-imaging area; and thesecond layer spanning the interconnections and barrier regions of thefirst layer only over the non-imaging areas and the second layeroverlays edges of the first layer.

The above and other objects of the present invention will become moreapparent when taken in conjunction with the following description anddrawings wherein identical reference numerals have been used, wherepossible, to designate identical elements that are common to thefigures.

Advantageous Effect of the Invention

The present invention has the advantage of a thin lightshield and aninterconnect metallization layer using a process that minimizescorrosion of the aluminum and TiW layers. A second advantage is that thepatterned second layer of a bi-layer metallization is not used as a maskfor the etch of the first layer of the bi-layer metallization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view in cross section of an image sensor of the presentinvention illustrating initial steps in producing the image sensor;

FIG. 2 is a drawing illustrating a step in the manufacturing processafter FIG. 1;

FIG. 3 is a drawing illustrating a step in the manufacturing processafter FIG. 2;

FIG. 4 is a drawing illustrating a step in the manufacturing processafter FIG. 3;

FIGS. 5 a and 5 b are an alternative embodiment of the presentinvention;

FIG. 6 is also an alternative embodiment of the present invention; and

FIG. 7 is a perspective view of a digital camera for illustrating atypical commercial embodiment to which the ordinary consumer isaccustomed.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a typical image sensor 10 consists of an array ofphotosensitive elements or pixels 30 in an image area. Within each pixelare regions that are exposed to light so that an electrical signal maybe created in response to the incident light. In addition, there areregions within the pixel, which are prevented from receiving lightbecause the light will degrade the imaging performance. A typical imagesensor 10 also provides dark reference pixels 32 that are insensitive tolight because they are covered with a light shield. The signal fromthese dark reference pixels 32 are used in the signal processing portionof the camera to indicate the signal of photosensitive pixels when nolight is incident upon them. In addition, interconnects are providedwithin the image sensor to electrically connect various parts of theimager and to provide means to connect the imager to external circuits.

Referring to FIG. 1, there is shown an initial stage of forming an imagesensor 10 of the present invention. This stage includes providing asubstrate 20 having a plurality of photosensitive sites 30 that convertincident light into charge packets. An insulator 40 spans and covers thesubstrate 20 and includes an opening 45 therethrough for forming acontact hole, and the first layer 60 of a bi-layer metallizationstructure is deposited on the insulator 40. In the preferred embodiment,the first layer of the bi-layer metallization is a titanium and tungstenalloy, and a bi-layer metallization is described. However other metalsor combination of metals and/or their compounds can be used. Theimportant properties of this first layer are that it is opaque in orderto be used as a light shield, and that the metal can be used as part ofa bi-layer metallization process where this first layer provides abarrier preventing the interaction of the silicon substrate with theupper and more conductive layer. Other first layers may be tungsten, ortungsten silicide, or molybdenum, or molybdenum silicide. Photoresist isselectively disposed on the TiW layer (not shown) to form a mask toprevent the etching of the underlying TiW layer. The exposed regions ofthe TiW layer are then etched using a fluorine-based plasma etchant.

Referring to FIG. 2, there is shown the resulting cross-section afterthe selective etching of the titanium and tungsten alloy layer 60, andthe removal of the photoresist. The titanium and tungsten alloy 60covers those regions of the pixel that should not be exposed to lightfor forming a light shield. The titanium and tungsten alloy 60 may alsocover dark reference pixels 32. In addition, the titanium and tungstenalloy 60 remains where metallization interconnects and bus lines,generally region 80, are to be provided. The present invention includesthe capability to separately pattern the titanium and tungsten alloy 60so that it may be used as a local interconnect to electrically connectdifferent parts of the imager that are not required to conduct highcurrent levels, or to connect parts of the imager that are very close toeach other, or other instances where the high conductivity of aluminumis not required. This local interconnect has the advantage of lowercapacitive coupling to other parts of the imager and its circuitrybecause the total interconnect height is less than the bi-layermetallization. An example is shown in FIG. 6. An aluminum layer 90 (seeFIG. 4) will be used in combination with the titanium and tungsten alloy60 as the interconnect for other circuits elements, as will be describedhereinbelow. A floating diffusion 100 is connected via the titanium andtungsten alloy 60 to a gate of a transistor 75 that forms a portion ofan image sensor output structure.

Referring to FIG. 3, after patterning the titanium and tungsten alloy60, a layer of aluminum or alloy of aluminum 90 such as an alloy ofaluminum and silicon, or an alloy of aluminum, silicon, and copper isdeposited. This aluminum alloy layer 90 covers the patterned titaniumand tungsten alloy 60 and the insulator 40 where the titanium andtungsten alloy 60 have been removed. It is noted for clarity that thecombination of the titanium and tungsten alloy 60 and the aluminum 90form the bus line 80.

Referring to FIG. 4, next, photoresist is selectively disposed spanningand covering the aluminum alloy layer 90 that is in the non-imagingareas, such as the dark reference pixels 32, and the interconnect regionor bus line 80. This photoresist then masks a chlorine-based etch of thealuminum alloy layer 90. The chlorine-based plasma etch selectivelyetches aluminum layer 90, but does not etch the titanium and tungstenalloy 60, nor the insulator 40. The photoresist is then removed. Thealuminum alloy 90 no longer covers the imaging area or photosensitivesite 30. The aluminum 90 does cover the titanium and tungsten alloy 60over interconnect region 80, and therefore forms the bi-metalinterconnect wiring used for bus lines and other electrical connections.In the bi-layer structure, the aluminum alloy 90 can cover both the topand the sides of the titanium and tungsten alloy 60 so that corrosion ofthe interconnect wiring is minimized. In particular, it is noted thatthe aluminum alloy covers the edges 95 of the titanium and tungstenalloy 60. The aluminum alloy 90 may also be patterned to cover the lightshielded dark reference pixels 32 with or without the underlyingtitanium and tungsten alloy layer 60 if the titanium and tungsten alloylayer 60 does not provide sufficient opacity in this region. Thealuminum alloy 90 does not cover the local interconnections made withthe titanium and tungsten alloy 60 only. For clarity of understanding,it is noted that the titanium and tungsten alloy 60 form a barrierregion to prevent intermixing between the gate region and the aluminumlayer 90 and to prevent intermixing of source and drain regions with thealuminum layer 90. The remaining steps needed for completion of acommercially usable image sensor are well known in the art and need notand will not be discussed in detail herein.

A second embodiment provides the same advantages, but instead of acontinuous bi-layer metallization, the titanium and tungsten alloy 60 ispatterned so that it is placed only in the contact holes and an overlaparound the contact holes. The overlap of the contact holes ensures thatthe contact hole is completely covered by the titanium and tungstenlayer within alignment variations of the process. The aluminum alloyalone is used for the interconnect layer in regions away from thecontact hole. In this embodiment, the conductivity of the interconnectis about the same as the first embodiment, and junction spiking andelectromigration at the contact holes is prevented by the barrier layer(the titanium and tungsten alloy), but reduces the thickness of themetallization interconnect over much of the device. FIGS. 5 a and 5 bshow an example where the titanium and tungsten alloy 60 are patternedto cover only the contact hole 105, while the aluminum alloy 90 alone isused in regions away from the contact hole 105.

Referring to FIG. 7, there is shown an electronic device, such as adigital camera 110, for illustrating a typical commercial embodiment forthe image sensor 10 of the present invention.

The invention has been described with reference to a preferredembodiment. However, it will be appreciated that variations andmodifications can be effected by a person of ordinary skill in the artwithout departing from the scope of the invention.

Parts List

-   10 image sensor-   20 substrate-   30 photosensitive elements (image area) or pixels-   32 dark reference pixels-   40 insulator-   45 opening-   60 titanium and tungsten alloy layer (1^(st) layer)-   75 transistor-   80 interconnect region or bus line-   90 aluminum alloy layer-   95 edges-   100 floating diffusion-   105 contact hole-   110 digital camera

1. An image sensor comprising: (a) a substrate having photosensitiveareas; (b) an insulator spanning the substrate; and (c) a first andsecond layer of a multi-layer metallization structure, wherein the firstlayer forms light shield regions over selected portions of thephotosensitive area as well forming circuit interconnections and barrierregions to prevent spiking into the substrate or gates at contacts inthe non-imaging area; and the second layer spanning the interconnectionsand barrier regions of the first layer only over the non-imaging areasand the second layer overlays edges of the first layer.
 2. The imagesensor as in claim 1, wherein the first layer is substantially TiW andthe second layer is substantially aluminum.
 3. A digital cameracomprising: an image sensor comprising: (a) a substrate havingphotosensitive areas; (b) an insulator spanning the substrate; and (c) afirst and second layer of a multi-layer metallization structure, whereinthe first layer forms light shield regions over selected portions of thephotosensitive area as well as forming circuit interconnections andbarrier regions to prevent spiking into the substrate or gates atcontacts in the non-imaging area; and the second layer spanning theinterconnections and barrier regions of the first layer only over thenon-imaging areas and the second layer overlays edges of the firstlayer.
 4. The digital camera as in claim 1, wherein the first layer issubstantially TiW, and the second layer is substantially aluminum.